The Sung capital at Jiang's Star is a very important station, lore-wise. Destroying it should not be easy, even for a player with endgame gear. There are several strategies to strengthen the Sung capital's defenses.
Hit Points & Regen
The most immediate way would be to increase the hit points and regeneration. Hit points buy time and regeneration also sets a bar for the player's power.
Dragon Slaver
The capital could use a dedicated Dragon Slaver guard. The ship would be a weaker version that does not use the Qianlong archcannon, or does not drop the weapon in identifiable or usable form. This would also introduce the player to the Dragon Slaver early. The Dragon Slaver should wait until the capital is partially damaged to show up as a surprise.
Cyberattacks
Sung ships specialize in cyberattacks and computing power. The capital could use highly aggressive cyberattacks originating from the outer defenses and the station itself, significantly weakening any player that tries to attack directly. Destroying the outer defenses would reduce the coverage and rate of cyberattacks.
Shipyards
A station as important as the capital would need a lot of reinforcements, but the Sung have yet to reveal their shipyards in the game. The outer defenses at the capital could become shipyards that build a capped number of Earth Slaver and Steel Slaver guards/patrols.
Phylactery
Thanks to the story, the player could acquire a special item that significantly weakens the capital, allowing them to destroy it as intended. This would probably involve a large number of slave coffins, a special cyber device, or maybe even the Qianlong archcannon.
After Destruction
Destroying the station would supposedly mark a turn in the Huari storyline. As a reward, the player could learn about the location of a secret system just like how they learn about Huaramarca.
Remember that in easier difficulty, player does more damage and takes less. Someone playing in Story mode can easily brute force through challenges that would be a challenge in Challenge mode mostly through enemies' hp to withstand attacks long enough.
For defenses, maybe Sung can have armor similar to worldship armor that is immune to tier 1 and 2 damage, and either highly resistant or immune to tier 3. If player has tier 4 weapons (that are level 9 or 10), he should blow holes through the armor. Make the armor dreadnought weight, so that none of George's playerships (so far) can use it if it drops.
As for Dragon Slaver, Qianlong seems to be its signature weapon - it should keep it. By Jiang's Star, player can get level 9 weapons like Ion9, Ferian cannon, or maybe a lucky spawn in an Adventurer's Outfitter shop. Any slavers outside of Huaramarca should not have the guaranteed damaged drop, just roll the dice which are not in the player's favor, like for other loot.
The Qianlong-related Huari missions which are only meant to occur in the Huaramarca system can be triggered in any system if a Qianlong is detected in that system.
Currently, because Dragon Slavers/Qianlongs are only created in the Huaramarca system, this never happens.
Additional Dragon Slavers, or more precisely, their wrecks, can trigger the find/repair Qianlong missions outside of Huaramarca.
A check for the Huaramarca system would need to be added to the mission code to prevent bugs.